The reflective LCD generally uses natural light as a light source rather than additional light source such as back-light.
The theory of the reflective LCD can be summarized by that a natural light is radiated from an upper substrate, and then the light is reflected via a reflecting plate disposed at a bottom position of a lower substrate., at this time, the light is absorbed or transmitted depending on the arrangement of liquid crystal molecules.
Most common twisted nematic (TN) mode reflective LCD has the drawback of narrow viewing angle. Therefore, conventionally the hybrid mode reflective LCD capable of displaying full color and having a fast response time in the low voltage condition has been suggested. However, the hybrid mode reflective LCD only uses the birefringence effect of the liquid crystal molecules, accordingly the contrast ratio is degraded since the gray scale inversion is easily occurred depending on the viewing direction. To solve foregoing problem, a bi-axial compensating film is applied to the hybrid mode reflective LCD. However, the bi-axial compensating film is difficult to produce.
Therefore, conventionally the reflective LCD without using any optical compensating film has been suggested to solve the problem of gray scale inversion and to obtain wide viewing angle.
As shown in FIGS. 1A and 1B, the reflective LCD comprises a lower substrate 11, an upper substrate 12 opposite to the lower substrate 11, a pixel electrode 13 formed on the lower substrate 11 in the form of strip, a counter electrode 14 formed at the same plane with the pixel electrode 13 and spaced apart by a selected distance, a first homeotropic alignment layer 19 coated on the upper substrate 12, a second homeotropic alignment layer 20 to cover the lower substrate 11 where the pixel electrode 13 and the counter electrode 14, and a liquid crystal layer 15 sandwiched between the first and second homeotropic alignment layers 19,20. Herein, although not shown in the drawing, a color filter is provided at an inner surface of the upper substrate 12.
The reflective LCD further comprises a reflecting plate 16 disposed at an outer surface of the lower substrate 11, a polarizing plate 18 disposed at the backside of the upper substrate 11 and a quarter wave plate 17 disposed between the upper substrate 11 and the polarizing plate 18. Herein, liquid crystal molecules of positive dielectric anisotropy are used for the liquid crystal layer 15. The polarizing plate 18 is disposed such that its polarizing axis makes 45 degrees with the direction of an electric field, and the quarter wave plate 17 is disposed such that its axis makes 45 degrees with the polarizing axis of the polarizing plate 18. Herein, the pixel electrode 13 can be made of an opaque material such as Al or Cr.
Operation of the reflective LCD as constituted above is as follows.
When no voltage is applied to the device, as shown in FIGS. 1A and 2A, all the liquid crystal molecules 15a of the liquid crystal layer 15 are arranged such that their long axes are perpendicular to faces of the substrates 11,12 according to the homeotropic alignment layers 19,20. Then this, light of a selected direction among not-polarized light of the light source is linearly-polarized in the left or right via the polarizing plate 18 once the light passing the polarizing plate 18 passes the quarter wave plate 17, the light is left-circularly-polarized or right-circularly-polarized. Since all the liquid crystal molecules 15a in the liquid crystal layer 15 are arranged in the z direction, the light passed the quarter wave plate 17 directly passes through the liquid crystal layer 15 without causing any phase difference in the light. The light arrived at the reflecting plate 16 changes its transmitting direction into the −z direction. Accordingly, before the left (or right)-circularly-polarized light is reflected, the light is right (or left)-circularly-polarized. The right (or left)-circularly-polarized light directly passes the liquid crystal display 15 and the light is radiated into the quarter wave plate 17 again. The light passed through the quarter wave plate 17 is left (or right)-linearly-polarized. However, since an axis of the light passed through the polarizing plate 18 is perpendicular to an axis of the light radiated to the polarizing plate 18, the light radiated from the quarter wave plate 17 to the polarizing plate 18 does not pass the polarizing plate 18. Accordingly, the screen shows dark state.
On the other hand, when voltage is applied to the device as shown in FIGS. 1B and 2B, the liquid crystal molecules 15a directly in contact with the substrates hold their arrangement same as when no voltage is applied to the device according to an influence of the homeotropic alignment layers 19,20. In the respective central regions of the pixel electrode 13 and the counter electrode 14 similar to the case when no voltage is applied thereto, long axes of the liquid crystal molecules 15a are arranged perpendicular to faces of the upper substrate 12 and the lower substrate 11. Since there is formed a horizontal electric field E1 and an elliptic electric field E2 between the pixel electrode 13 and the counter electrode 14, the liquid crystal molecules are arranged according to electric fields E1,E2. Accordingly, there are formed two domains forming a symmetry in their left side and right side with respect to the center line of the pixel electrode 13 and the counter electrode 14 except the region in contact with the substrates 11,12.
With regard to the light transmitting process, light of a selected direction among not-polarized light of the light source is linearly-polarized by passing through the polarizing plate 18. Once the light passes the quarter wave plate 17, the linearly-polarized light is changed to be left (or right)-circularly-polarized. According to the angle between the optical axes of the liquid crystal molecules and a transmitting axis of the quarter wave plate 17, the light changes its polarizing state while passing the liquid crystal layer 15. That is to say, the left-circularly-polarized light is left-linearly-polarized while passing the liquid crystal layer 15 and the light is left (or right)-linearly-polarized again. The light reflected from the reflecting plate 16 is right (or left)-linearly-polarized. The right (or left)-circularly-polarized light is left (or right)-circularly-polarized while passing again the liquid crystal layer 15. Afterward, the left (or right)-circularly-polarized light is radiated into the quarter wave plate 17. The light passed through the quarter wave plate 17 is right (or left)-linearly-polarized. Consequently, the axis of the polarizing plate 18 is coincided with the optical axis passed through the quarter wave plate 17. Accordingly, the screen shows white state.
To form an in-plane electric field E1 and an elliptic electric field E2 as described above, the distance l between the electrodes 13,14 should be set relatively larger than the cell gap d, and widths of the respective electrodes 13,14 should be set relative wider, for example in the range of 10˜20 μm to obtain a selected degree of intensity of electric field. However, when the device has a constitution as described above, although an electric field almost parallel to the substrate is formed between the electrodes 13,14, there are occurred equipotential lines in the upper portions of the electrodes 13,14 since most of the upper portions of the electrodes 13,14 are not affected by the electric field. As a result, the transmittance is degraded-greatly.